Most processing devices have some form of an Operating System (OS). A typical OS has two-modes of operation: a user-mode of operation, and a kernel-mode of operation.
The kernel-mode of operation: has unrestricted access to the underlying hardware for the device on which it processes; can access every available memory address with pretty much unrestricted access; and can execute any processor instructions associated with the device on which the OS operates.
The user-mode of operation is the primary mode where users access and perform operations via applications, but these operations have limited ability to access memory or directly access underlying hardware. Operations in user-mode use Application Programming Interface (API) system calls, some of which can process in kernel-mode (using kernel operations) to perform needed hardware operations and memory accesses where user access is restricted.
During operation of a processing device, the user's processes will jump in and out of user-mode and kernel-mode based on API system calls.
So, when an OS kernel is patched a reboot is typically required of the processing device (such as a server having multiple processors). The reboot ensures consistency between processes jumping in an out of the kernel-mode by stopping all processes. So, a reboot can be a costly and disruptive operation, which is hard to schedule particularly when many parties, via independent devices, depend on the server to be available. Moreover, processors for devices being serviced by the server have to be stopped and usually for a non-trivial extended period of time. Some other approaches can achieve a patch without a reboot; these solutions typically stop all processors running the OS while the patch takes place to ensure consistency. So, either approach necessitates stopping the processors of the OS to install a patch, which makes the OS unavailable to users during the patching process.
It is noted the problem also exists with standalone devices and not just multi processors of a server. For example, many users will attest to the frustration with updating their phone OS because during the update their phones are completely unusable. The same holds true for set-top boxes, tablets, laptops, computer desktops and the like.